Lutetium oxyorthosilicate (LSO) or Lu2SiO5, invented by present co-inventor Charles L. Melcher and described in U.S. Pat. No. 4,958,080, incorporated by reference herein in its entirety, is a well-known crystal scintillator material that is widely used for gamma-ray detection in PET as well as other applications.
LSO is typically doped with 0.05-0.5% Ce, while controlling other impurities at low levels. The development of cerium-doped LSO in the early 1990s represented a significant advance in inorganic scintillators for medical imaging. With its 7.4 g/cm3 density, high light yield, and fast decay time (˜45 ns), LSO is widely regarded as the best detector material available for PET. Since the time that LSO went into large-scale commercial production in the late 1990s there has been a significant effort by the present inventors and others to improve the scintillation properties of LSO.
In particular, the conventional method of LSO crystal growth used cerium doping alone (i.e. without any codopant), at relatively high cerium concentrations. Additionally, LSO growth methods typically utilized a crystal growth atmosphere consisting primarily of nitrogen, which resulted in undesired formation of oxygen vacancies and other defects associated with insufficient amounts of oxygen. It is also known that the light yield of LSO crystals as grown using prior art methods is on average significantly lower than the theoretical maximum, and the decay time of these crystals tended to vary.
Additionally, there exists a need for improvement in the decay time of LSO, especially in light of new techniques developed for image data acquisition such as Time-Of-Flight PET (TOF-PET). Further, so-called “phoswich” (or phosphorescence sandwich) detectors that often suffer from mismatched light outputs, mismatched indices of refraction, or the undesired absorption of scintillation light from one scintillator by the other, could be improved by such improved LSO scintillators.
The scintillation properties of LSO grown under such conditions can vary significantly from boule to boule, and in different parts of the same boule, which consequently increased the cost of commercial crystal production caused by the large number of out-of-spec crystals produced.
Considerable work has been done in recent years by a number of researchers on the use of codoping to improve the scintillation properties or growth of various inorganic scintillators, including doping of gallium garnets with divalent elements in order to suppress spiral growth, or with tetravalent dopants to decrease absorption loss. Y3Al5O12:Ce has been doped with Ca2+ to control oxygen vacancies, as described by Rotman et al. in J. Appl. Phys. vol. 71, no. 3, pp. 1209-1214, February 1992, incorporated herein by reference. Both divalent and tetravalent dopants have been used in the growth of LuAlO3:Ce, as described by Derdzyan et al. in Nucl. Instr. Meth. Phys. Res. A. 537, pp. 200-202 (2005), incorporated herein by reference. LSO:Ce has been doped with 0.02% Ca2+ or Mg2+, as described in Zavartsev et al, “Czochralski growth and characterization of large Ce3+:Lu2SiO5 single crystals co-doped with Mg2+ or Ca2+ or Tb3+ for scintillators,” J. Crystal Growth vol. 275, pp. e2167-e2171, 2005, incorporated herein by reference. Zavartsev et al. reported some improvement in light yield, though no change in decay time, relative to LSO:Ce with no co-doping.
Zagumennyi et al., U.S. Pat. No. 7,132,060, incorporated herein by reference, disclosed a scintillation substance composition in the form of a single crystal represented by the formula CexLu2+2y−x−zAzSi1−yO5+y, where A is an element selected from Gd, Sc, Y, La, Eu, Tb and Ca, and where x is between 1×10−4 and 0.2 f.u., y is between 0.024 and 0.09 f.u., and z is between 1×10−4 and 0.05 f.u (it is noted that the disclosed formula is not enabled for Ca as the resulting compound would not be charge balanced).
There remains a need in the art for further improvements in the growth of LSO crystals, for example, to improve coincidence timing characteristics in PET imaging applications, to increase the scintillation light yield and improve light yield uniformity, improve energy resolution, achieve the ability to modify scintillation decay time as desired for particular applications and improve decay time uniformity, compensate for negative effects of lattice defects and traps, and to reduce or eliminate the occurrence of undesired color centers.